1. Field of the Invention
This invention relates generally to the prevention of wind-induced damage to building structures and specifically to a method and apparatus for reducing the lifting forces which act on a rooftop due to the winds.
2. Description of the Prior Art
High wind gusts from typhoons, hurricanes, tropical storms, tornados and severe thunderstorms can wreak significant damage to building structures. In addition to missile hazards created when wind gusts cause debris to become airborne, wind damage may occur due to essentially laminar wind flow over the top of a building structure which creates lift and shear forces that can cause a roof to separate from the building.
A good discussion of the applicable aerodynamics is contained in John D. Anderson, Jr., Introduction to Flight, McGraw-Hill Publishing Company (3d ed. 1989). Simplistically put, due to the laws of conservation of energy and momentum, when an object is placed into a flow field, the mass flow (and hence the product of the density, velocity and area of flow) in the flow field at the windward and leeward side of the object are equal. This relation ship is referred to as the continuity equation. When the flow field is able to move smoothly around the object, the generally parallel (laminar) incident streamlines (lines which are everywhere parallel to the direction of fluid flow at a given instant) expand to envelope the object and collapse downstream of the object to form generally parallel laminar streamlines again. Such flow is referred to as attached flow. Thus, when the wind flows around a building, the streamlines flowing up over the roof cover a greater distance than the streamlines flowing along the side of the building. For attached flow, the flow over the roof may have a greater velocity than the flow along the side of the building, which, according to Bernoulli's equation, results in reduced pressure above the rooftop and resultant lift forces acting on the rooftop. Additionally, the wind creates shear forces at the roof surface due to friction. The lift and shear forces on the rooftop may cause the roof to separate from the building.
Numerous proposals exist to reduce the lift forces which act on rooftops. For example, U.S. Pat. No. 2,270,537 and U.S. Pat. No. 2,270,538 issued to Ludington, U.S. Pat. No. 3,280,524 issued to Hull, U.S. Pat. No. 3,828,498 issued to Jones, U.S. Pat. No. 4,005,557 issued to Kramer et al., and U.S. Pat. No. 5,918,423 issued to Ponder all disclose apparatus for mounting to a rooftop to cause flow separation at the rooftop. Flow separation, where the streamlines separate from the rooftop surface and form eddies and sometimes random, disorganized flow, results from a severe adverse pressure gradient and generally causes a drastic loss of overall lift. Such apparatus are known as spoilers.
Occasionally, flow separation results in well defined vortices. When the vortices flow close to a rooftop surface, the low pressure caused by the vortices can create localized suction forces which can damage rooftops. Particularly problematic are winds incident to a building at its corner which tends to create vortices along two generally orthogonal axes across a flat rooftop. U.S. Pat. No. 6,601,348 issued to Banks et al. discloses airfoil-shaped members for mounting to the windward of a rooftop which are designed to interrupt the formation of vortices along a rooftop.
3. Identification of Objects of the Invention
A primary object of the invention is to provide a method and apparatus for reducing lift forces which act on a rooftop by creating downward-facing lift forces.
Another object of the invention is to provide a method and apparatus for reducing lift forces which act on a rooftop which is effective regardless of the wind direction.
Another object of the invention is to provide a method and apparatus for reducing lift forces which act on a rooftop which is esthetically pleasing.
Another object of the invention is to provide a low cost method and apparatus for reducing lift forces which act on a rooftop.
Another object of the invention is to provide a method and apparatus for reducing lift forces which is suitable for use on both flat rooftops and gabled rooftops.
Another object of the invention is to provide a method and apparatus for reducing lift forces on trailers, semi-trailers, mobile homes, freight containers, and vehicles.